V. A. Trepakov

Electron paramagnetic resonance studies of manganese centers in SrTiO3: Non-Kramers Mn3+ ions and spin-spin coupled Mn4+ dimers

An X- and Q-band electron paramagnetic resonance (EPR) study is reported on SrTiO3 single crystals doped with 0.5 at. % MnO. The EPR spectra originating from the Su2009=u20092 ground state of the Mn3+ ions are shown to belong to three distinct types of Jahn-Teller centers. The ordering of the oxygen vacancies due to the reduction treatment of the samples and the consequent formation of an oxygen vacancy associated with the Mn3+ centers is explained in terms of the localized charge compensation. The EPR spectra of the SrTiO3:Mn crystals show the presence of next-nearest-neighbor exchange coupled Mn4+ pairs in the 〈110〉 directions.

Tensile strain induced changes in the optical spectra of SrTiO3 epitaxial thin films

Effect of biaxial tensile strains on optical function and band edge transitions of ultra thin epitaxial films was studied using as an example a 13 nm thick SrTiO3 films deposited on KTaO3 (100) single-crystal substrates. Optical functions in the 200–1200 nm spectral range were determined by spectroscopic ellipsometry technique. It was found that tensile strains result in a shift of the low energy band gap optical transitions to higher energies and decrease the refractive index in the visible region. Comparison of the optical spectra for strained SrTiO3 films and for homoepitaxial strain-free SrTiO3: Cr (0.01 at %) films deposited on SrTiO3 (100) single crystalline substrates showed that this “blue” shift of the band gap could not be related to technological imperfections or to reduced thickness. The observed effect is connected with changes in the lowest conduction and in the top valence bands that are due to increase of the in-plane lattice constant and/or onset of the polar phase in the tensile strain-induced ultra-thin epitaxial SrTiO3 films.

Photochromism and polaronic photocharge localization in diluted KTA1-xNbxO3

Ultraviolet (UV)-light-induced optical absorption in the near infrared (NIR) region was observed in diluted KTa1−xNbxO3 single crystals (x=0,0.004,0.007,0.012,0.07) at low temperatures. Illumination by wideband light (3.10–4.13eV, 300–400nm) is accompanied by the appearance of a broad NIR absorption band with the position of the maxima varying in the 0.69–0.8eV (1.54–1.79μ, T=1.3K) region for different Nb concentrations. This UV-light-induced absorption is absent in nominally pure KTaO3, as well as in all Nb diluted specimens at elevated temperatures. The centers responsible for the photochromic NIR absorption bands are tied to interband optical transitions of pair Nb4+ electronic polarons. The photochromic experimental data, supplemented by luminescence studies in the visible range, evidence the strong localization of the photocharge carriers by pair Nb4+ polarons at low temperatures. It is suggested that namely the strong localization of the photocarriers plays a crucial role in photoinduced gigantic di...

Pulse-electron paramagnetic resonance of Cr3+ centers in SrTiO3

Electron paramagnetic resonance on chromium doped SrTiO3 samples grown using the Verneuil technique shows the presence of charge-compensated Cr3+-VO as one of the dominant chromium centers. The spin-lattice relaxation processes have been investigated in samples with both isotropic Cr3+ and Cr3+-VO centers in heavily doped SrTiO3. The relaxation of longitudinal magnetization was dominated by the sum of two exponentials with two time constants (i.e., a slow and a fast constant) at liquid-helium temperatures. The results of fitting the temperature variation of T1 suggest that the dominant exponential contribution is related to the spin-phonon relaxation time arising from the local phonon mode.

Polar state in a SrTiO3-KTaO3 solid solution

Dielectric properties of ceramic samples of a 0.85SrTiO3-0.15KTaO3 solid solution have been studied. The temperature dependence of the dielectric permittivity exhibits maxima with a strong frequency dispersion, which is described by the Vogel-Fulcher law with a characteristic temperature Tg=26 K. These features are apparently connected with transition to a polar glasslike state.

Contribution of spontaneous polarization and its fluctuations to refraction of light in ferroelectrics

The expressions for the spontaneous polar contribution δnis to the principal values of the refractive index due to the quadratic electro-optic effect in ferroelectrics have been considered within the phenomenological approach taking into account the polarization fluctuations. A method has been proposed for calculating the magnitude and temperature dependence of the root-mean-square fluctuations of the polarization (short-range local polar order) Psh = 〈Pfl2〉1/2 below the ferroelectric transition temperature Tc from temperature changes in the spontaneous polar contribution δnis (T) if the average spontaneous polarization Ps = 〈P〉 characterizing the long-range order is determined from independent measurements (for example, from dielectric hysteresis loops). For the case of isotropic fluctuations, the proposed method has made it possible to calculate Psh and Ps only from refractometric measurements. It has been shown that, upon interferometric measurements, the method developed in this work allows calculating Psh and Ps directly from the measured temperature and electric-field changes in the relative optical path (the specific optical retardation) of the light.

Dielectric permittivity and phase transitions in the SrTiO3-KTaO3 system

The dielectric permittivity and losses of the (SrTiO3)0.85(KTaO3)0.15 solid solution have revealed an unusual behavior at temperatures of 5–300 K and frequencies ranging from 100 Hz to 1 MHz. The dielectric permittivity exhibits a broad maximum at ≈40 K, which obeys the Curie-Weiss law in the high-temperature wing, and the onset of low-temperature relaxation contributions. Besides, while for T>40 K it is a fully ergodic, uniformly ordering system, the uniform polar ordering is replaced with decreasing temperature by two low-temperature phase transitions to a glass-like state. The proposed mechanism for the dielectric relaxation and the observed phase transformations involves reorientation and ordering in the system of electron and hole polarons, which form in the course of charge compensation of the heterovalent ions Sr2+, K+ and Ti4+, Ta5+ distributed randomly in the sublattices.

Optical absorption spectra and energy levels of Er3+ ions in KTaO3 crystals

The optical absorption spectra of erbium-doped potassium tantalate (KTaO3:Er) crystals have been studied. Absorption lines corresponding to the intraconfiguration f-f electron transitions from the 4I15/2 ground state to the levels of 4F9/2, 4S3/2,2H11/2, 4F7/2, 4F5/2(4F3/2), 2H9/2, and 4G11/2 excited states of Er3+ ions have been observed in a wavelength range of 350–660 nm. The transitions to 4F9/2, 2H11/2, 4F7/2, and 2H9/2 levels have been studied in detail at T = 2 K. Based on the obtained results, it is suggested that Er3+ impurity centers of two types exist in KTaO3:Er3+ crystals. The number of lines observed for the major type centers corresponds to that theoretically possible for Er3+ ions in a noncubic crystal field with a local compensation of the impurity charge. The energy positions of the levels of excited states for the observed transitions are determined.

Study of absorption spectra of Er3+ in KTaO3 crystals

The first results of the study of optical absorption spectra of KTaO3: Er3+ crystals are presented. In the 350–660-nm region, lines are observed deriving from intraconfigurational electronic transitions from the 4I15/2 ground state to levels of the 4F9/2, 4S3/2, 2H11/2, 4F7/2, 4F5/2(4F3/2), 2G9/2, and 4G11/2 excited states of the Er3+ ions. A comprehensive study of transitions to the 4F9/2, 4S3/2, 2H11/2, and 4F7/2 levels at 77 K is carried out. The number of lines observed for the above transitions fits the theoretically possible number for ƒ-ƒ electronic transitions in Er3+ ions in the cubic crystal field. In the case of a differently charged substituted ion, this situation occurs only under nonlocal impurity charge compensation. The energies of the excited state levels for the transitions under study are determined.

Effect of external electric field on R line absorption spectra in LiNbO3:Cr3+ crystals

The effect of an external electric field on the R absorption lines of LiNbO3:Cr3+ crystals has been studied by a high-sensitivity differential technique at 77 K. Linear Stark effect has been observed in a field directed along the trigonal crystal axis. No effect of the field was observed when directed perpendicular to this axis. These characteristics of the Stark effect provide a convincing argument for the electric dipole moments of Cr3+ centers being oriented along the trigonal C3 axis of the crystal and indicate that the chromium centers in LiNbO3 have C3 symmetry.